Friday, October 25, 2013

Dielectric Spectroscopy

Thanks to the support and generosity of Al Stiegman, we have set up a platform for performing dielectric spectroscopy in the range of 50 MHz to 20 GHz using a vector network analyzer (VNA) and a 7 mm beadless coaxial air guide. The VNA allows for measuring reflected (S11 & S22) and transmitted (S12 & S21) microwave radiation, and based on an accurate calibration of the VNA and its cables, real and imaginary components of the permittivity and permeability can be obtained. These S-matrix parameters describe the microwave transport in the test material, and allow for the examination of absorption due to dipolar loss mechanisms in the material. This microwave spectroscopy is of interest in characterizing dielectric constants in frequency range of the VNA for electronics design purposes. For some materials, such as microwave-assisted catalysts, this microwave absorption is a fundamental materials design parameter. Microwave absorption is also an important probe of water containing materials, and as a fundamental probe of dipolar interactions in metal oxides and composites.

One constraint is that samples must fit the 7 mm coaxial wave guide. Solid materials such as the Teflon shown in this picture, can be machined to a 3 mm ID and 7 mm OD to tightly fit the coaxial guide. Deviations from the optimal ID and OD will require gap corrections. Powder samples need to be cast into toroids by mixing with wax or a resin.

A variety of software options are available. Some use various combinations of S-matrix parameters depending upon the sample configuration. For example, a material on a known short or dielectric backing is best suited to use of reflection data (S11 or S22). Some approaches can yield both permittivity and permeability information, while some approaches require specific sample length and positioning constraints. What VNA calibrations are required is also determined by the approach used. A complete calibration kit is a available to perform open, short, through, and high and low bandpass termination calibrations of the VNA and its cabling.

Data provided by this technique is complex permittivity, e = e' + ie", as a function of frequency, and complex permeability, u = u' + iu", as a function of frequency.  This is often expressed as a function of loss tangent, e"/e' and u"/u'.

Magneto-Transport in MPMS

CMMP has the capability of doing magneto-transport measurements in the Quantum Design MPMS SQUID magnetometer. Samples can be mounted on probes that allow the samples to be mounted either perpendicular or parallel to the applied magnetic field. Up to ten electrical connections are available for contacting, and can be assigned across multiple samples. Currently CMMP only has cabling for four contacts, but this is not an inherent limitation. Samples are generally covered with Teflon or Kapton tape to prevent wires from contacting from getting caught upon insertion into the MPMS sample tube.

DLL's have been written that allow for 2-pt resistance, 4-pt resistance, DC voltage and AC voltage using a multimeter.  These DLL's can be called from within the MPMS MultiVu software, allowing the MPMS to act as a magneto-transport platform. The multimeter can be used in conjunction with a lock-in for very low signal measurements with appropriate current-limiting resistors.

The sample above is simply a piece of niobium wire connected for 4-point resistance measurements. The data left shows resistance versus temperature through the Tc region.

PPMS EDC Shell

The Quantum Design MultiVu software for the PPMS allows GPIB interfaced devices to be triggered through "advisories" called within the PPMS scripting language. These advisories are referenced only by a number, and as such, "Set Advise Number 205" in the PPMS script may functionally mean "do a 4-point resistivity measurement".  It's much like ordering fast-food: give me a number 205.

These advisories are managed through a Windows OS queuing system, and are interfaced with the GPIB interfaced test equipment using a LabView EDC Shell developed at CMMP. This EDC Shell allows users to drop in sub-VI's designed to accomplish their measurement tasks right into the shell, allowing the Windows OS queuing and handshaking with the PPMS controller to be in the background.

PPMS Sample Pucks


These are optical photos of the PPMS resistivity pucks. The CMMP group has three of these so that samples can be contacted while one is in the system. Each puck has three stations that include four contacts for transport measurements. These contacts can be accessed either by our break-out box or the PPMS controller's resistivity bridge depending upon the application.

The large gold square is a heat sink to provide thermal conductivity to the PPMS temperature annulus. Ideally the samples should be heat-sunk to this block for ideal temperature control. While this is practical if the samples have insulating substrates, it should be noted that this surface is also electrically grounded. In the case of semiconducting or conducting sample substrates a piece of Kapton tape or other insulator should be placed under the sample.  That is what has bee done in the present case.

Contacting samples is an art, and CMMP will not do that for you. Conducting silver or graphite paints can be used, as well as indium, indium solder, or a wire bonder. In many cases forming good contacts becomes a material science issue as contacts need to be annealed, or surface oxides need to be removed. A test station allows the integrity of the contacts to be tested at room temperature using the test electronics to be used in one's experiment.

The proper placement of contacts also depends upon other factors such as the type of measurement one wishes to accomplish.  In cases of an irregular sample, the contacts might be best placed on the four corners to allow for a van der Pauw approach.  In the case of a Hall measurement of carrier densities, one might wish to even pattern the sample into a Hall bar and attach contacts accordingly.

The availability of three sets of contacts, labelled I+, I-, V+ and V- allows one to mount three samples for traditional 4-point transport measurements.  One need not assign the contacts to I+, I-, etc., unless one is using the PPMS resistivity bridge.  In other cases, one is limited only by one's imagination.   One can wire channel A for 4-point resistance, channel B for 2-point resistance on two samples, and channel C for a Hall measurement. One can use channel A for one's sample, B for a Hall device to measure applied field exactly, and channel C for a thermometer.  In more complex devices one can simultaneously measure longitudinal and transverse voltage drops across samples patterned into bars with multiple pairs of contacts using several channels to assign all of the necessary contacts to be accessed by the breakout box.

Quantum Design PPMS

Thanks to the kindness and support of Theo Siegrist we have been able to offer routine magneto-transport measurements using a Quantum Design PPMS. The PPMS is much like its magnetometry sibling, the MPMS (discussed in the CMMP SQUID magnetometry blog) in terms of field and temperature control. The temperature range is 1.8-400K, and the maximum field is 9T. Unlike the MPMS, there are no SQUID devices and gradiometer for moment detection. The PPMS, in its current configuration, is entirely a field and temperature platform for transport measurements. Like the MPMS, the PPMS has a scripting language that allows users to quickly write programs controlling their measurements.

The only test equipment intrinsic to the PPMS is a resistance bridge that is built into the PPMS controller. If has three measurement channels, and allows for 2-point and 4-point DC and AC resistance measurements. This is the standard PPMS "resistivity" option, which allows each of the three channels to be turned on and off within the PPMS control sequence, with the resistivity measured within the user defined parameters of maximum voltage, current and power. Resistance is converted to resistivity through user specified sample information. An internal calibration resistance load can be used to calibrate the bridge channels in a used-defined fashion.

In many cases this level of control does not produce the level of finesse desired. Test electronics dedicated to the PPMS allow for measurements beyond the capacity of the PPMS resistivity bridge, or with a high degree of control. These include a one-channel and two-channel lock-in, multimeter (capable of 2-point and 4-point resistance, as well as reading lock-in out-put voltage), source meter, and an LCR meter for capacitance and inductance measurements.

A break-out box allows each of the three channels on the PPMS resistivity pucks to be accessed using BNC or banana connectors, and there are accommodations for grounds to be isolated or shared with the break-out box for noise reduction.

Users commonly bring their own hardware and are invited to do so, though his hardware remains as a common resource.